watchdog: meson: remove magic value for reboot

This patch removes the magic value used for rebooting the board. This
value is useless and leads to a static checker warning as reported by
Dan Carpenter.

Signed-off-by: Carlo Caione <carlo@caione.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>